The isoenzymes and hybrid isoenzymes of mammalian pyruvate kinase will be examined to extend existing information on their properties, the way their physiological roles differ from one another, and their individual tissue distributions. An attempt will be made to establish whether a concerted or sequential model better explains their kinetic behavior, particularly of the hybrids formed from skeletal muscle and liver pyruvate kinase, as the first of these two species exhibits hyperbolic kinetics with substrates whereas liver pyruvate kinase is an allosteric enzyme.